When the NaK-ATPase complexes with ouabain or digoxigenin were formed in the presence of Na ion, Mg2 ion, and ATP, the pH did not influence their association rate constants and bindings, but the dissociation rate constants of ouabain were reduced with the increase of pH. When such complexes were formed in the presence of Mg2 ion and Pi, their association rate constants of ouabain and digoxigenin decreased with the increase of pH. While the equilibrated binding with ouabain did not change with pH, the apparent site number of the Scatchard plot of digoxigenin binding decreased with the increase of pH. Such pH effects on the association rate constants and the apparent site numbers corresponded to the change of levels of phosphoprotein by ATP or Pi. Another observation on the complex formed in the presence of Mg2 ion and Pi was that the dissociation rate constants of the complexes with ouabain and digoxigenin under pH 8.0 were constant, but they decreased significantly at about pH 8.0 and remained constant above pH 8.0.